Soulmates is an interesting concept. It's been here since the beginning of history, the maximum being 3 mates. If you have them, you are considered normal. If you don't have them you are considered unnatural. That's how common they were.
You would receive the marks at the young age of five. Only strong emotions came through unformed bonds. For example, even if you had never met your mate, you would feel anger, sadness, fear, and comfort through the bond sometimes. Of course only when they were strong though.
Once you found your mate(s) you would feel pain the closer you got until the bond was accepted. Although if rejected, you die a lot sooner then the average lifespan.
If the bond is accepted then one of you will stop ageing till the other gets within 5 years. Although soulmates are common, few will achieve the level that is called 'linked'. There is only two recorded times of someone being able to do that and has never been talked about since.
Linked is where you are able to see what your soulmate is seeing, for short periods of time. It is talked about everywhere and considered to be a myth, because the only recorded times were hundreds of years ago.